The instant invention relates to oven temperature control, and more particularly to electronic controllers and temperature sensors for controlling same.
The maintenance of a consistent cooking temperature has long been a problem which has plagued appliance engineers, gourmet chiefs, and homemakers alike. The variation of temperature within the oven cavity from the center to the walls, as well as the variation of temperature over time during a cooking cycle may result in inconsistent cooking behavior. As an example, cake which should be cooked at 350xc2x0 F. for 20 to 25 minutes may be nearly burned one time at 18 minutes, and may still be wet after 28 minutes another time. This problem is a result of many factors including the size of the oven cavity, the placement of the temperature sensor within the cavity, the type of heat source (gas versus electric), the amount of insulation used in the construction of the oven, convection currents within the cavity, as well as the sensor characteristics themselves.
Recognizing that some of these factors are beyond the appliance engineer""s control, efforts were made to design a mechanism of temperature sensing and burner control which would minimize temperature variation within the cavity due to controller induced changes. In the past, oven temperature controllers utilized electromechanical controls which included at least one hot thermostat within the cavity which controlled the relays or solenoids which supplied the fuel (gas or electric) to adjust oven temperature. This control was a simple ON/OFF type control which operated the burners to maintain the sensed temperature within a hysteresis band defined, in large part, by the hysteresis of the temperature sensing element itself. Such a temperature control band is illustrated in FIG. 6.
As may be seen with reference to FIG. 6, the set point line 10 indicates the desired temperature as set by the user. However, because the temperature sensor and control included hysteresis, the oven temperature control would actually not turn the burners off until the temperature had risen beyond the set point by a given hysteresis amount as illustrated by the line 12. Once the burner control had turned off the burners, the interior temperature within the oven begin to fall. Unfortunately, due once again to hysteresis of the temperature sensing and control circuitry, the interior temperature would be allowed to fall beyond the set point 10 to a point along the line 14. Once the temperature had fallen below line 14, the burners would again be turned on and the temperature would begin to rise. This temperature rise would continue until line 12 was reached, and the cycle would continue. The temperature hysteresis band of these early oven temperature controls was typically as wide as 20xc2x0 F., and was fairly constant for all temperature settings.
As electronic controls were introduced to appliance design, the operating characteristics of the electromechanical temperature control, including the hysteresis band, were emulated within the electronic controller. As with their electromechanical counterparts, a linear hysteresis band of approximately 20 F. was used throughout the set point band defined by line 10 of FIG. 6. Unfortunately, utilizing a linear hysteresis band results in a large percent error at lower cooking temperatures, e.g. a 20  F. band at the 170xc2x0 F. setting equates to a percent effort of +/xe2x88x926%, while at the setting of 550xc2x0 F. it equates to only a +/xe2x88x922% error.
Recognizing this large disparity in the percentage error resulting from emulating the electromechanical sensors of the past, the next generation of electronic oven temperature controllers utilized a stepped turn on hysteresis limit 16 as illustrated in FIG. 7. This stepped lower limit 16 allowed for the percent error allowed over the entire cooking cycle to be lowered to a more acceptable level. These next generation electronic controllers utilized three (3) to four (4) discrete lower limits as illustrated by line segments, 16A, 16B, and 16C, resulting in three to four discrete hysteresis bands. Typically, these bands were set to 5xc2x0 F., 10xc2x0 F., and 15xc2x0 F. for a three zone implementation, and to 5xc2x0 F., 10xc2x0 F., 15xc2x0 F., and 20xc2x0 F. for a four zone implementation. These discrete hysteresis zones greatly improved the cooking performance of the ovens in which these controllers were installed, especially when cooking delicate foods such as pastries, etc.
However, the non-linear nature of this lower hysteresis limit has also resulted in cooking control problems. Specifically, since a discontinuity exists between different cooking zones (e.g. defined by the upper hysteresis limit 12 and the first segment 16A, the upper limit 12 and the second segment 16B, and the upper limit 12 and the third segment 16C), inconsistent cooking performance was observed when the oven was set at a temperature near the end point of two zones. This inconsistent cooking performance is a result of the controller oscillating between the two adjacent control zones of lower limit 16. Attempts to stabilize this problem through software coding have met with limited success due to the limited code space available and the cost restraints imposed by the highly competitive appliance industry. As a result, this problem remains.
In addition to this problem, these next generation electronic controllers also suffer from a similar problem relating to initial turn on of the oven. When the oven is first turned on and a temperature is set by the user, the cavity temperature begins to climb. It is known in the oven art that the oven temperature will continue to climb once the burners are turned off during this initial pre-heat phase as illustrated by temperature curve 20 of FIG. 9. Because of this effect, the controller utilizes a separate preheat turn off limit as illustrated in FIG. 8 as line 18P or 18NP. The position of this preheat turn off limit 18P or 18NP in relation to the normal control hysteresis limits 12, 16 shown in FIG. 7 varies depending on many factors, including whether the oven is a pyro type (see line 18P) or a non-pyro type (see line 18NP).
Because a pyro type oven includes a self-cleaning cycle which raises the interior temperature to approximately 900xc2x0 F., it includes much more insulation than a non-pyro type oven which does not include a self cleaning cycle. Because of this increased insulation, the pre-heat turn off limit 18P is typically lower than the pre-heat turn off limit 18NP in a non-pyro oven, and may be below the steady state burner turn on limit 16 of FIG. 7. In a non-pyro type oven, the pre-heat turn off limit 18NP may actually be above the steady state turn off limit 12 shown in FIG. 7 to allow for the increased need to heat the walls of the oven (which contain relatively little insulation compared to a pyro type oven). In any event, the pre-heat turn off limit 18P or 18NP is set to minimize temperature overshoot and maximize temperature settling time within the steady state temperature control band 12, 16 of FIG. 7.
However, the non-linear nature of this pre-heat turn off limit 18P or 18NP has also resulted in cooking control problems. Specifically, since a discontinuity exists between different cooking zones (e.g. in FIG. 7 defined by the upper hysteresis limit 12 and the first segment 16A, the upper limit 12 and the second segment 16B, and the upper limit 12 and the third segment 16C), the pre-heat limit 18P or 18NP was also discontinuous. These discontinuities also resulted in inconsistent pre-heating performance when the oven was first turned on and set at a temperature near the end point of two zones. As with the above, this inconsistent pre-heating performance is a result of the controller oscillating between the two adjacent pre-heat zones of lower limit 18P or 18NP. Attempts to stabilize this problem through software coding have met with limited success, also due to the limited code space available and the cost restraints imposed by the highly competitive appliance industry. As a result, this problem also remains.
It is therefore an object of the invention to overcome these and other problems existing in the art. More particularly, it is an object of the instant invention to provide a new and improved electronic controller for oven temperature control which overcomes the above and other problems existing in the art. Specifically, it is an object of the instant invention to provide an electronic oven temperature controller which minimizes the software coding and expense for temperature control while increasing the performance and consistency of the temperature control. It is a further object of the instant invention to allow for individual oven characterization of the temperature control limits. It is an additional object of the instant invention to allow for re-characterization of the temperature control limits, both in absolute value and in shape. It is a further object of the instant invention to allow for re-programming of the temperature control limits.
These and other aims, objectives, and features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.